Nitrogen oxide (NO.sub.x, for example, N.sub.2 O, NO.sub.x, NO.sub.2, etc.) generated from combustion processes is a serious atmospheric pollutant. In fact, continuous on-line monitoring of NO.sub.x from combustion processes is often necessary to meet strict regulations of the U.S. Clean Air Act, which are expected to become more and more stringent in the future. Furthermore, because the amount of NO.sub.x in the exhaust of a combustion process is indicative of the air/fuel ratio, NO.sub.x concentration can also be used for feedback control of the air-to-fuel ratio of the combustion process in order to achieve optimal fuel efficiency.
Various apparatus and techniques are known in the art for determining the concentration of NO.sub.x in a gas mixture, which may include, for instance, gaseous oxygen (O.sub.2), nitrogen (N.sub.2), and/or other gases. Typically, the electrochemical sensing of gaseous oxide compounds has been based on a well known "oxygen pumping principle," which is described briefly hereafter. The oxygen pumping principle has been widely publicized and is described in, for example, U.S. Pat. No. 4,005,001 to Pebler, U.S. Pat. No. 4,770,760 to Noda et al., U.S. Pat. No. 4,927,517 to Mizutani et al., U.S. Pat. No. 4,950,380 to Kurosawa et al., U.S. Pat. No. 5,034,107 to Wang et al. and U.S. Pat. No. 5,034,112 to Murase et al.
Generally, a solid electrolyte conductive to oxygen ions is utilized when employing the oxygen pumping principle. The electrolyte is commonly zirconia (ZrO.sub.2), bismuth oxide (Bi.sub.2 O.sub.3), ZrO.sub.2 and/or Bi.sub.2 O.sub.3 containing alkaline earth dopants, such as calcia (CaO), or containing rare earth dopants, such as yttria (Y.sub.2 O.sub.3), as a stabilizer, or some other suitable electrolyte having the properties more fully described hereafter. These electrolytes show a high permeability (conductance) to oxygen ions O.sup.2- when biased at a constant voltage and when maintained above a certain temperature, for instance, greater than 200.degree. C. in many applications. In other words, in an environment containing oxygen, these electrolytes can selectively permit oxygen to pass therethrough if certain biasing and temperature conditions are met. Said another way, these electrolytes exhibit high conductivity at elevated temperatures, and application of a voltage creates an O.sup.2- current or flux.
In sensors utilizing these oxygen-ion-permeable electrolytes, electrocatalysts are usually disposed on opposing sides of the electrolyte, and a voltage is applied across the electrolyte via the electrocatalysts. The electrocatalysts typically comprise platinum (Pt), rhodium (Rh) and/or other noble metals. In this configuration, the combination of the electrocatalysts and the electrolyte disposed therebetween forms an electrochemical cell which is often referred to as a "pumping cell" because it pumps oxygen from the gas mixture exposed to the pumping cell. The pumping cell causes oxygen in the gas mixture to be reduced to oxygen ions O.sup.2- at the negative electrocatalyst (cathode), and then the oxygen ions O.sup.2- move through the electrolyte to the positive electrocatalyst (anode), where they are oxidized to oxygen again and discharged.
Numerous techniques have been proposed in the art for determining the amount of oxygen and/or oxide compounds in the environment around electrochemical cells, particularly pumping cells, by monitoring the voltage and/or current generated across and/or through the electrolyte. A brief discussion of several exemplary types of prior art sensors is set forth hereafter, but it should be noted that this discussion is not exhaustive.
One type of sensor is described in U.S. Pat. No. 5,217,588 to Wang. This sensor employs two electrochemical cells on a zirconian electrolyte. One cell senses only oxygen gas and the other cell senses all the gases which contain oxygen, including the oxygen gas. Both electrochemical cells are exposed to the same gas mixture, and the difference between the sensed signals is a measure of the concentration of NO.sub.x in the gas mixture.
Another type of sensor is described in U.S. Pat. No. 5,034,112 to Murase et al. In this sensor, an electrocatalyst for reducing NO.sub.x is placed on an electrolyte adjacent to a pumping cell. A current is induced in the pumping cell so as to control the oxygen concentration in the environment around the pumping cell. When the oxygen concentration is depleted to a predetermined level, the electrocatalyst supposedly begins to deplete NO.sub.x, and the concentration of NO.sub.x is determined by measuring the current supplied to the pumping cell.
Although the sensors of the prior art have some merit, they do not provide for highly accurate measurement of NO.sub.x or other oxide compounds in gas mixtures because the electrocatalysts utilized for the electrochemical cells do not provide for sufficient selectivity between oxygen and NO.sub.x. In other words, some amounts of oxygen and some amounts of these oxide compounds are undesirably consumed by the wrong electrocatalyst, and this phenomenon results in inaccurate measurements of oxygen as well as NO.sub.x concentrations. Moreover, if the gas mixture contains a relatively low NO.sub.x concentration as compared with that of oxygen, the signal-to-noise ratio is small, and an accurate determination of the NO.sub.x concentration is even more difficult. In exhaust gases or emissions produced by internal combustion engines or furnaces, the concentration of oxygen is typically several thousand times higher than the NO.sub.x concentration. Hence, measurements of NO.sub.x in exhaust gases using the prior art techniques are undesirably and unavoidably inaccurate.